1. Field of the Invention
This invention relates to the field of processes and apparatus for the high pressure polymerization of ethylene alone or together with minor amounts of one or more other copolymerizable monomers to form polyethylene homopolymers and copolymers, respectively. As used herein, the terms "polyethylene", "ethylene polymer" and terms of like import shall be understood to include homopolymers and copolymers of ethylene.
2. Description of the Prior Art
Various proposals have heretofore been made for the continuous high pressure polymerization of ethylene in a series of reactors, either with the object of improving overall conversion efficiency and/or to provide polymers having certain desirable properties.
U.K. Pat. No. 765,501 describes the polymerization of ethylenically unsaturated monomers, including ethylene, at elevated pressure and temperature in the presence of free radical catalysts in a series of reactors. Thus, for example, ethylene is polymerized in the first reactor to convert from 10 to 25% of the initial ethylene feed to polyethylene. The effluent from the first reactor is then deactivated such as by cooling in a heat exchanger. In the deactivated state, additional catalyst is added to the reaction mass, the reaction mass is reheated to effect its reactivation and the reactivated reaction mass is then introduced into the second reactor to accomplish further conversion of monomer. The sequence of deactivation, catalyst addition, reactivation and polymerization is repeated for the remaining reactors in the series until the desired amount of conversion of ethylene to polyethylene is attained. The polyethylene produced in each reactor can be recovered separately therefrom or the polymer can be totally collected following the final stage of reaction.
U.S. Pat. No. 2,964,514 to Fawcett describes a process for polymerizing olefins such as ethylene in a series of autoclave reactors having a heat exchanger positioned between pairs of reactors. Effluent from the first reactor at a temperature of about 140.degree. C. is passed through the cooler and exits therefrom at about 110.degree. C. precipitating the polymer. The reaction stream, now in the form of a slurry containing ethylene polymer and unreacted ethylene monomer, is introduced into the second reactor wherein further polymerization takes place.
U.S. Pat. No. 3,380,978 to Ryan et al. relates to the preparation of polyolefins such as polyethylene in a series of reactors, the first of which is a short holdup tubular reactor and the second of which is a longer holdup constant environment autoclave reactor. Control of polymer properties is achieved principally by imposing limitations on the tubular and autoclave reactor temperatures and pressures, reactant feed temperature, catalyst concentration, point of catalyst entry, contact times and composition of monomer feed.
U.S. Pat. No. 3,875,128 to Suzuki et al. describes a process and apparatus for polymerizing ethylene in a battery of autoclave reactors connected by means of piping to a heat exchanger disposed between the reactors. The pressure of the reaction effluent from the first reactor is reduced through a let-down valve and the reduced pressure reaction stream is passed through the cooler to reduce the temperature of the stream to above 120.degree. C. but lower than the reaction temperature in the first reactor by at least 20.degree. C. The cooled reaction stream is then introduced into the second reactor to continue the polymerization reaction. In the similar process and apparatus for polymerizing ethylene shown in U.S. Pat. No. 3,875,134, ethylene is first polymerized in an autoclave reactor, and after passage through a pressure let-down valve and a heat exchanger, the reaction stream is introduced into a tubular reactor for completion of the polymerization reaction.
While it has been observed that cooling the reaction medium passing from one reactor to another in a series of ethylene high pressure polymerization reactors leads to significantly greater levels of monomer conversion, the reduction in pressure of the reaction stream from the first, or preceding, reactor prior to passage of the stream through a heat exchanger as in the processes and apparatus of U.S. Pat. Nos. 3,875,128 and 3,875,134 tends to result in phase separation, or precipitation, of the polyethylene from the reaction stream with consequent fouling or plugging of the heat exchanger. The increased maintenance and reactor down-time imposed by the need to periodically clear the heat exchanger of accumulated ethylene polymer constitutes a significant practical and economic disadvantage for reactor systems operating in the aforesaid manner. There has thus existed a need for an ethylene polymerization process and apparatus which benefits from the high conversion rates characteristic of multiple reactor systems, but which is far less susceptible to problems of polymer phase separation and heat exchanger fouling than the multiple reactors heretofore proposed or used.